Calculator Inputs
Example Data Table
| Boat Type | LWL | Coefficient | Estimated Hull Speed | Typical Use |
|---|---|---|---|---|
| Small Daysailer | 18 ft | 1.34 | 5.69 knots | Short coastal sailing |
| Coastal Cruiser | 30 ft | 1.34 | 7.34 knots | Weekend passages |
| Offshore Cruiser | 40 ft | 1.34 | 8.47 knots | Long passages |
| Light Racer | 36 ft | 1.40 | 8.40 knots | Performance sailing |
Formula Used
Hull Speed: Hull Speed = C × √LWL
C is the hull speed coefficient. The common value is 1.34. LWL is the load waterline length in feet. The answer is in knots.
Speed Length Ratio: SLR = Speed in knots ÷ √LWL in feet
Froude Number: Fn = Speed in meters per second ÷ √(g × LWL in meters)
Wave Length Estimate: Wave Length = (Speed ÷ C)²
Passage Time: Time = Distance in nautical miles ÷ Speed in knots
Displacement Length Ratio: D/L = Displacement long tons ÷ (0.01 × LWL in feet)³
How to Use This Calculator
- Enter the boat name or keep the sample name.
- Enter the load waterline length.
- Select the correct length unit.
- Keep 1.34 for a standard displacement estimate.
- Change the coefficient only for known hull forms.
- Enter passage distance in nautical miles.
- Enter a target speed, or leave it blank.
- Add displacement in pounds if you want D/L ratio.
- Press Calculate to view results below the header.
- Use CSV or PDF buttons to save results.
Understanding Sailboat Hull Speed
Hull speed is a useful estimate for displacement sailboats. It shows the speed where the bow wave and stern wave nearly match the waterline length. At that point, the boat begins climbing its own wave system. Extra sail force may bring only small gains. The estimate does not replace sea trials. It gives a practical planning limit for many cruising hulls. Record wind strength, reefing state, engine use, and sea condition beside each observed run. These notes make future estimates more accurate for your vessel later.
Why Waterline Length Matters
Longer waterline length usually means higher potential speed. A longer hull creates a longer wave before reaching the same wave pattern. That is why two boats with similar rigs can feel different underway. The longer boat often carries speed more easily. Waterline length can also change with heel, trim, and load.
Using Results Onboard
The calculator gives hull speed in knots and common land units. It also shows a speed length ratio. This ratio helps compare boats of different sizes. A value near the chosen coefficient marks the classic displacement limit. The travel time estimate helps plan passages. It is still wise to add allowance for tide, current, waves, leeway, and traffic.
Advanced Checks
A target speed can be entered for comparison. When the target is above hull speed, the result warns that the boat may need surfing, planing, strong current, or unusual conditions. The wave length estimate shows the deep water wave connected with the selected speed. If displacement is provided, the displacement length ratio gives context for heavy or light designs.
Good Data Gives Better Answers
Use measured load waterline when possible. Design length may differ from the actual floating length. Enter consistent units. Pick the standard coefficient for general estimates. Change it only when you understand the hull form. Slender racing hulls, multihulls, planing hulls, and foiling craft may exceed this rule. Heavy cruisers may perform below it in rough seas. The calculator should guide decisions, not replace seamanship. Use it with logbook data. Compare predicted speed with real passages. Over time, your own boat will reveal a realistic cruising speed range. That range is often more useful than a single maximum number.
FAQs
What is sailboat hull speed?
Hull speed is an estimated speed limit for many displacement sailboats. It occurs when the boat moves between its bow and stern wave pattern.
Why does the calculator use waterline length?
Waterline length controls the wave length a displacement hull creates. Longer waterline length usually supports a higher estimated hull speed.
Is 1.34 always the best coefficient?
No. It is a common classic value. Some hulls may use a higher or lower value because of shape, weight, and sailing conditions.
Can a sailboat exceed hull speed?
Yes. Surfing, planing, strong current, light displacement, or special hull forms can exceed the estimate. The rule mainly fits displacement hulls.
What is speed length ratio?
Speed length ratio compares speed with waterline length. It helps compare boats of different sizes using a simple dimensionless style reference.
What is Froude number?
Froude number compares boat speed with gravity wave behavior. It is useful in naval architecture and model testing.
Should I enter displacement?
Displacement is optional. Enter it if you want the displacement length ratio. That ratio gives extra context about heavy or light designs.
Can I use this for multihulls?
You can use it for a rough reference. Many multihulls do not follow classic displacement hull speed limits in the same way.